astrophysics

Chandra Probes the Depths

X-Ray Observatory Finds Universe Filled with Black Holes

By Hong Ma

In over two years of observations, NASA’s Chandra X-ray Observatory has brought back countless images of never-before-seen stellar activity and a treasure trove of information about the exotic world of black holes, supernovae, quasars, and neutron stars -- whose behaviors are often much stranger than science fiction.

After 20 years in development Chandra’s successful launch and operation has made history in many ways. At 23 tons, it is one of the heaviest objects ever carried by shuttle. The crew was led by Eileen Collins, the first female ever to command a shuttle. Chandra’s July 1999 release into deep space, led by astronaut Catherine G. Coleman ’83, marked the 50th flight into space by an MIT graduate.

MIT’s Center for Space Research (CSR) built two of the four science instruments on Chandra -- the High Energy Transmission Grating (HETG) spectrometer and the Advanced CCD Imaging Spectrometer (ACIS), jointly developed by MIT and Pennsylvania State University. The combination of these two instruments gives Chandra orders of magnitude improvements in resolution in both imaging and spectroscopy over previous x-ray observatories.

“We are seeing images ten times shaper and spectra 1000 times sharper [than in previous missions],” said Claude R. Canizares, the principal investigator for HETG and director of CSR. The new capabilities make it possible to actually resolve the dynamics of interstellar processes. Sharper spectra help to accurately measure temperature, chemical composition, and even relative velocities of stellar objects.

Chandra’s remarkable resolution is useful for observing the behavior of a black hole, the extraordinarily dense object predicted by Einstein’s general theory of relativity, whose gravitational force is so strong that even light cannot escape. Gases sucked into black holes, however, are heated to extremely high temperatures and emit powerful x-rays. By analyzing the x-ray luminosity, astronomers are able to put a lower limit on the mass of the black hole. Chandra allows astronomers to observe the rich variety of black holes, which span nine orders of magnitude in size. The size of the observed black holes ranges from stellar black holes of a few solar masses to massive black holes found at the center of quasars, which weigh in at 109 solar masses.

Chandra has been able to resolve the haze of background x-ray radiation seen by previous instruments into discreet sources, many of which are black holes. This observation has effectively assigned new demographics for black holes in the universe. Whereas previously fewer than 40 black holes had been observed, Chandra’s new data has shown that there may be as many as 300 billion black holes in the observable universe.

By resolving fainter and fainter objects in the background, Chandra “has the ability to resolve the edge of the observable universe. You don’t need a bigger telescope. We are very close right now and just need to do a few more scans,” said Canizares. “The observable universe is finite because the universe has a finite age and a finite rate of expansion.”

Chandra has obtained some of the most magnificent images of a supernova, the final stage of a star’s life.

“These x-ray images show the details of the transfer of energy from a rotating neutron star out into the interstellar medium. You can see rings and jets and material moving along the jets and spreading around the rings in all sorts of the details that we were never able to see before,” said Harvey Tananbaum, director of the Chandra X-ray Observatory Center in Cambridge. By resolving the dynamics of the remnants, it is possible to get to the details of the physics of supernovae where heavy elements like silicon and iron are ejected.

Chandra observations providing insight into the great cosmic mystery of gamma ray bursts -- the mysterious explosions in space that can radiate energy equivalent to the entire rest mass of a star in a few seconds. After each explosion, remnants become a slowly-decaying x-ray source. Because of its sensitivity, Chandra can trace the fall-off of x-ray afterglows for periods of weeks or months, whereas previous x-ray telescopes could only track these after glows for a few days. Chandra’s spectrometer has also picked up spectral lines of surprisingly cool iron that may provide geometric information on the explosion.

Chandra is also involved in finding the amount and distribution of matter in the universe. In the regions between the galaxies are vast quantities of hot gas that emit a lot of x-rays. The quantity of matter in these regions of space can be measured by first finding the density of the gases from their intensity and the temperature of the gases from the spectrum. Knowing intensity and temperature, it is possible to find the pressure, which can in turn be used to find gravitational pull necessary to keep the gas from spreading apart. By finding out the distribution of mass in the universe, it maybe possible to answer questions about whether there is dark matter -- particles that interact only by means of gravity -- in the universe, or will the universe continue to expand forever or end in a “big crunch?”

Chandra is actually the third installment of NASA’s fleet of great space observatories covering much of the electromagnetic spectrum. They include the Hubble Space Telescope, the now retired Compton Gamma Ray Observatory, and the Space Infrared Telescope Facility, which is scheduled for launch in July 2002. By combining data collected by these instruments and other satellites from Europe and Japan, it is possible to answer some of the most basic questions of the universe.

“Right now we are at an incredibly rich time when there is a lot of new information coming in about the big bang, the size and age of the universe, the amount of matter in the universe, whether the universe will expand forever or contract, whether there is dark matter and dark energy,” said Canizares.